This invention relates generally to coatings for substrates. More particularly, it is directed to methods for applying protective coatings on substrates employed in high temperature.
High-performance alloys are often the materials of choice for various components which are exposed to high-temperature environments. As an example, turbine blades and other parts of turbine engines are often formed of nickel-based superalloys because they need to maintain their integrity at temperatures of at least about 1000xc2x0 C.-1150xc2x0 C. In many instances, the alloys have to be covered with protective coatings which provide greater resistance to corrosion and oxidation at high temperatures, as compared to the alloy itself.
A common example of a protective coating material for turbine engine blades is a metal-aluminide, such as platinum aluminide. This type of material is usually deposited in several steps. For instance, the platinum is electroplated onto the blade, using P-salt or Q-salt electroplating solutions. In the second step, the platinum layer is diffusion-treated with aluminum vapor to form platinum aluminide.
When such a protective coating becomes worn or damaged, it must be carefully repaired, since direct exposure of the underlying substrate to excessive temperature may eventually cause the component to fail and adversely affect various parts of the engine. The coatings often have to be repaired several times during the lifetime of the component. The xe2x80x9coverhaulxe2x80x9d of the protective coating usually involves complete removal of the coating, followed by the application of a new coat of the material.
In many situations, certain portions (i.e., xe2x80x9clocal areasxe2x80x9d) of the protective coating require repair, while the remainder of the coating remains intact. As an example, portions of a platinum-aluminide protective coating may become depleted in aluminumxe2x80x94especially when the component is exposed to an oxidizing atmosphere for an extended period of time. In the case of superalloy substrates, loss of aluminum from the adjacent protective coating can be detrimental to the integrity of the superalloy since, under desirable circumstances, the coating provides thermal oxidation protection at elevated use temperatures.
For selective, local repair, the traditional process of completely removing the platinum-aluminide coating is often inefficient. Such a process requires multiple electroplating steps, followed by an aluminiding process, such as, pack aluminiding. In addition to being labor-intensive and time-consuming, the traditional repair process can sometimes be detrimental to the component. For example, the repeated stripping and re-coating of protective layers may damage the substrate, xe2x80x9ceatingxe2x80x9d into its thickness and thereby changing critical dimensions of the component.
Thus, new techniques for applying metal-aluminide coatings to substrates would be welcome in the art. The techniques should be efficient and not labor-intensive. They should also be useful for coating selected portions of the substrate, such as, only those sections which require repair. Moreover, the new techniques should preserve the integrity of the substrate surface.
In one aspect, this invention is directed to a method for forming a metal-containing layer on a substrate, comprising the following steps:
(a) depositing a slurry of the metal on the substrate; and
(b) heating the metal slurry under temperature and time conditions sufficient to remove substantially all volatile material from the slurry, and to form a layer which comprises the metal.
The slurry of the metal is deposited on the substrate by various techniques, such as spraying. For purposes of this invention, the metal slurry layer is separate from aluminum metal or the aluminum slurry layer. It may be applied in multiple applications, with heat treatments between each application, to remove the volatile components. The layer of metal is then usually subjected to a diffusion heat treatment. A typical metal used in this process is platinum. The substrate is often a superalloy, such as, a nickel-based superalloy used in a turbine engine.
In another embodiment, a slurry of aluminum is deposited over the slurry of the metal, before or after the metal has been heat-treated to some degree. A diffusion heat treatment after the deposition of the aluminum results in a coating which comprises a metal-aluminide compound. The coating may also comprise a compound based on aluminide, metal and at least one metallic element in the substrate.
In still another alternative, the deposition of the metal slurry is followed by a volatile-removing heat treatment, and then a diffusion heat treatment. Aluminum is then deposited by a diffusion heat treatment of aluminum vapor.
The invention is also directed to a method for repairing a damaged or worn metal-aluminide coating applied over a substrate, comprising the following steps:
(i) removing the damaged or worn coating from a selected area on the substrate; and then
(ii) applying additional coating material over the selected area, by
(I) depositing aluminum and a slurry of the metal on the selected area, in place of the coating removed in step (i); and
(II) heating the aluminum and the slurry under temperature and time conditions sufficient to remove substantially all volatile material from the deposited aluminum and slurry, and to form a layer of metal-aluminide on the selected area. The aluminum is usually in the form of an aluminum slurry deposited on the selected area after the deposition of the slurry of the metal (such as, platinum). This method is a convenient technique for quickly and effectively repairing platinum-aluminide coatings, such as those which are not easily accessible for other repair techniques.
Related articles of manufacture are also described below, along with further details regarding the various features of this invention.